In a conventional multi-coin validator, coins pass along a path past a number of spaced sensor coils which are each energised to produce an inductive coupling of the coin. The degree of interaction between the coil and the coin is a function of the relative size of the coin and coil, the material from which the coin is made and also its surface characteristics. Thus, by monitoring the change in impedance presented by each coil, data indicative of the coin under test can be provided. The data can be compared with information stored in the memory to determine coin denomination and authenticity. An example of such an inductive validator is disclosed in UK Patent Specification 2 169 429 (Coin Controls Limited).
It has also been proposed in the past to use optical detectors to discriminate between coins on the basis of their optical characteristics e.g. diameter.
However, it has been found that certain types of counterfeit coins such as lead discs or discs made from a lead alloy cannot readily be distinguished by conventional validators from true coins since the characteristics of the counterfeits are too similar to acceptable coins to permit effective discrimination.
Coin validators incorporating piezo electric elements for sensing the impact of the coin falling onto part of the coin path are known from International Patent Application No. PCT/DK82/00072 (GNT Automatic A/S) and GB-A-2 236 609 (Mars Incorporated).
In PCT/DK82/00072, the mechanical elasticity of the coin is determined by causing the coin to apply an impact force to a piezo electric element, the output of which is amplified and compared with a preset threshhold amplitude value to determine coin acceptability. A coin made of lead or lead alloy produces a lower output of the piezo electric device than a corresponding true coin and thus by appropriately setting the threshhold value, discrimination between true and counterfeit coins can be achieved.
GB-A-2 236 609 discloses a similar arrangement in which the signal from the piezo electric element is differentiated prior to comparison with a predetermined threshhold in order to reduce the effects of variation in coin impact velocity.
It is an object of the present invention to provide an improved coin discrimination apparatus utilising an impact responsive sensor.